CN107078327B

CN107078327B - Film

Info

Publication number: CN107078327B
Application number: CN201580041972.8A
Authority: CN
Inventors: D·琼斯; J·罗兹尔; S·卡瓦利尔; S·苏比亚恩托; S·伯顿
Original assignee: Guo Jiakeyanzhongxin; Universite de Montpellier I; Johnson Matthey Fuel Cells Ltd
Current assignee: National Research Center; Universite Montpellier 2 Sciences et Techniques; Johnson Matthey Hydrogen Technologies Ltd
Priority date: 2014-08-04
Filing date: 2015-08-04
Publication date: 2020-09-22
Anticipated expiration: 2035-08-04
Also published as: KR102473497B1; JP2017532716A; EP3177388A1; DK3177388T3; US10777833B2; GB201413794D0; JP6707519B2; US20170279142A1; WO2016020668A1; CN107078327A; KR20170038881A; EP3177388B1

Abstract

Disclosed is an electrolyte membrane comprising: (i) a porous cluster of nanofibers, wherein the nanofibers are comprised of a non-ion conducting heterocyclic-based polymer comprising basic functional groups and being soluble in an organic solvent; and (ii) an ion-conducting polymer which is a partially or fully fluorinated sulfonic acid polymer; wherein the porous clusters are substantially completely impregnated with the ion-conducting polymer, and wherein the thickness of the porous clusters in the electrolyte membrane is distributed over at least 80% of the thickness of the electrolyte membrane. Such membranes may be used in proton exchange membrane fuel cells or electrolysers.

Description

Film

Technical Field

The present invention relates to an electrolyte membrane and its use in electrochemical devices, in particular its use in proton exchange membrane fuel cells.

Background

A fuel cell is an electrochemical cell comprising two electrodes separated by an electrolyte. A fuel, such as hydrogen, an alcohol, such as methanol or ethanol, or formic acid, is supplied to the anode, and an oxidant, such as oxygen or air, is supplied to the cathode. Electrochemical reactions occur at the electrodes and convert the chemical energy of the fuel and oxidant into electrical energy and heat. An electrocatalyst is used to promote the electrochemical oxidation of the fuel at the anode and the electrochemical reduction of oxygen at the cathode.

Fuel cells are generally classified according to the nature of the electrolyte used. Typically the electrolyte is a solid polymer membrane, wherein the membrane is electronically insulating, but ionically conductive. In Proton Exchange Membrane Fuel Cells (PEMFCs), the membrane is proton conductive, and protons produced at the anode are transported across the membrane to the cathode, where they combine with oxygen to form water.

The main component of a PEMFC is a Membrane Electrode Assembly (MEA), which essentially consists of five layers. The central layer is a polymeric ion-conducting membrane. On either side of the ion-conducting membrane there is an electrocatalyst layer containing an electrocatalyst designed for the specific electrolysis reaction. Finally, adjacent to each electrocatalyst layer is a gas diffusion layer, which is porous and electrically conductive and allows the reactants to reach the electrocatalyst layer and conduct the current generated by the electrochemical reactions.

Conventional ion-conducting membranes used in PEMFCs are typically formed from sulfonated perfluorinated polymeric materials, often collectively referred to as perfluorinated sulfonic acid (PFSA) ionomers. As an alternative to PFSA-type ionomers, ion-conducting membranes based on partially fluorinated or non-fluorinated hydrocarbon sulfonated or phosphonated polymers may be used. Recent developments in PEMFCs require thinner membranes (<50 μm) and higher Ion Exchange Capacity (IEC) or lower Equivalent Weight (EW) due to the advantages obtained (improved ion conductivity, improved water transport, etc.) and therefore, in order to provide the mechanical properties required to increase resistance to premature failure, a reinforcement, typically expanded polytetrafluoroethylene (ePTFE), is embedded within the membrane.

While such reinforced membranes often have lower proton conductivity than non-reinforced membranes of the same thickness, the improvement in mechanical properties enables the use of thinner membranes of lower electrical resistance.

Other types of reinforcing agents have also been proposed, as disclosed in WO 2011/149732.

Disclosure of Invention

While reinforced membranes such as those described above allow for the use of thinner membranes while maintaining mechanical strength, deficiencies still exist. In particular, limitations can be seen in practical operation, where humidity conditions can vary considerably from fairly high levels (e.g., starting at cold conditions) to fairly dry levels (operating at maximum rated power density) over a short period of time, where the membrane can degrade to higher levels than is acceptable. In an accelerated stress test designed to simulate and accelerate such an operation, the wet/dry cycle accelerated stress test causes swelling/debulking of the membrane so that these membrane degradation effects can be observed more rapidly.

It is an object of the present invention to provide an improved electrolyte membrane suitable for use in PEMFC and PEM electrolyzers.

The present invention provides an electrolyte membrane comprising:

(i) a porous mat (mat) of nanofibers, wherein the nanofibers are comprised of a non-ion conducting heterocyclic polymer comprising basic functional groups and being soluble in an organic solvent; and

(ii) an ion-conducting polymer which is a partially or fully fluorinated sulfonic acid polymer;

wherein the porous clusters are substantially completely impregnated with the ion-conducting polymer, and wherein the thickness of the porous clusters in the electrolyte membrane is distributed over at least 80% of the thickness of the electrolyte membrane.

Drawings

FIG. 1: schematic of the electrolyte membrane of the invention.

FIG. 2: SEM images of electrospun Polybenzimidazole (PBI) nanofiber clusters and fiber size distributions of the examples.

FIG. 3: effect of using solvent sheath on PBI fiber size distribution.

FIG. 4: cross-sectional SEM of the membrane of example 1.

FIG. 5: the OCV fixed fraction of MEA 1 and MEA 3 was tested (hold test) at 85 ℃ and 13% RH.

FIG. 6: at OCV, wet-dry cycles for MEA 1, MEA 2, and MEA 3.

FIG. 7: MEA 4, MEA 5 and MEA 6 during the stack durability test.

Detailed Description

Preferred and/or optional features of the invention will now be set out. Any aspect of the invention may be combined with any other aspect of the invention unless the context requires otherwise. Any preferred or optional features of any aspect may be combined with any aspect of the invention, alone or in combination, unless the context requires otherwise.

The present invention provides an electrolyte membrane comprising a porous cluster of nanofibers substantially completely impregnated with an ion conducting polymer.

The porous clusters provide mechanical reinforcement to the electrolyte membrane.

The porous clusters are formed from entangled nanofibers of a non-ion conducting heterocyclic-based polymer containing basic functional groups. The heterocyclic polymer is soluble in organic solvents, and in particular the polymer is soluble in N-methylpyrrolidone (NMP), Dimethylformamide (DMF), dimethylacetamide (DMAc) or Dimethylsulfoxide (DMSO), suitably DMAc or DMSO, preferably DMAc.

The average diameter of the nanofibers is suitably 100-400nm, suitably 100-300nm, preferably 150-250 nm.

The length of the nanofibers is not important to the present invention, but each nanofiber should be long enough (e.g., a few centimeters) to be entangled with one or more other nanofibers or with itself.

The nanofibres are suitably spun nanofibres, i.e. the nanofibres are formed using spinning techniques. Examples of suitable spinning techniques include, but are not limited to, electrospinning and forced spinning.

Heterocyclic polymers, preferably basic heterocyclic polymers, include polybenzimidazole, poly (pyridine), poly (pyrimidine), polybenzothiazole, polyoxadiazole, polyquinoline, polyquinoxaline, polythiadiazole, polytriazole, polyoxazoles and polythiazoles and derivatives thereof. Suitably, the polymer is a functionalized polyazole or zwitterionic polyazole, such as polybenzimidazole, polytriazole, polythiazole and polydithiazole and derivatives thereof; most suitably polybenzimidazole.

Suitably, the nanofibres are formed from a single heterocyclic-based polymer and are not a blend of two or more heterocyclic-based polymers.

The heterocyclic polymer may also be crosslinked; i.e. one polymer chain is bonded to another polymer chain. Crosslinking may improve the mechanical stability of the electrolyte membrane.

The heterocyclic polymers may also have intrinsic free radical scavenging properties. Such properties would be beneficial in electrolyte membranes and provide protection against chemical degradation mechanisms such as damage from peroxy radical species. This will therefore also serve to provide a more durable membrane. The use of heterocyclic polymers having such properties will also eliminate the need to add additional materials having radical scavenging properties or hydrogen peroxide decomposition catalysts such as cerium cations, ceria, manganese dioxide or other additives to the electrolyte membrane, thus avoiding the drawbacks associated with the incorporation of these materials.

The porous cluster has an open structure and a porosity of 70-98%, suitably 80-95%, suitably 85-95%, preferably 90-95%. The porosity is determined by the ratio of the volumetric mass of the porous cluster (determined by its geometric size and its mass) to the known polymer density.

The average basis weight of the porous clusters was 1g/m²-7g/m²Suitably 1.5g/m²-3g/m²。

The porous clusters in the electrolyte membrane are suitably at a maximum thickness of 50 μm, 30 μm, suitably 25 μm, preferably 20 μm.

A suitable minimum thickness of the porous clusters in the electrolyte membrane is 5 μm, suitably 10 μm.

To form the porous mat, the nanofibers are formed on a suitable substrate or surface, suitably by a spinning technique. For example, the nanofibers can be formed using electrospinning: an electrospinning solution of a heterocyclic based polymer contained in a suitable solvent is pushed through a needle using a syringe pump and a high voltage is applied to the needle. The clusters of electrospun nanofibers are collected on a grounded rotating drum collector that moves both translationally and rotationally, the collector being disposed at a distance from the needle, for example, about 10-15cm from the needle. Fiber morphology is obtained by controlling solution parameters such as concentration, while cluster thickness and uniformity are controlled by deposition time and collector rotation/translation speed.

The porous mat is not subjected to any additional processing, for example any densification processing such as calendering or welding or the like.

The ion-conducting polymer is suitably a proton-conducting polymer, in particular a partially or fully fluorinated sulphonic acid polymer. Examples of suitable proton conducting polymers include perfluorosulfonic acid ionomers (e.g., perfluorosulfonic acid ionomers)

(E.I.DuPont de Nemours and Co.)、

(Asahi Kasei)、

(SolvaySpecialty Polymer)、

(Asahi Glass Co.)。

The porous clusters are substantially completely impregnated with the ion-conducting polymer to form an electrolyte membrane. By "substantially completely impregnated" is meant that at least 80%, suitably at least 90%, suitably at least 95%, and ideally 100% of the pores of the porous cluster are filled with the ionically conductive polymer.

Suitably, an excess of ion-conducting polymer is present on both surfaces of the electrolyte membrane to aid adhesion to the catalyst layers.

The porous clusters may be impregnated with the ion-conducting polymer by:

the ion-conducting polymer layer (in solution/dispersion) is cast onto a support material. While the ion-conducting polymer layer is still wet, porous nanofiber clusters are laid into the wet layer and the ion-conducting polymer is impregnated into one face of the porous clusters. Another ionically conductive polymer layer is applied to a second face of the porous cluster and is immersed into the porous cluster from the second face. The impregnated porous mat is dried and appropriately annealed to form an electrolyte membrane.

The solution/dispersion of the ionically conductive polymer may comprise additional components, such as short nanofibers, for example 1-50 μm short nanofibers.

Alternative methods of impregnating porous clusters with ion conducting polymers will be known to those skilled in the art.

In the final electrolyte membrane of the invention, the weight ratio of ion-conducting polymer to nanofibres is suitably greater than 70:30, preferably greater than 90: 10. Suitably, the ratio of ionically conductive polymer to nanofibers is less than 98: 2. In this context, nanofibers refer to nanofibers in a porous cluster.

The thickness of the porous clusters in the electrolyte membrane is suitably distributed over at least 80% of the thickness of the final electrolyte membrane, suitably at least 85% of the thickness, most suitably at least 90% of the thickness. The porous clusters are distributed across the thickness of the membrane such that the thickness of the electrolyte membrane and the thickness of the porous clusters are substantially equal; in practice, however, the thickness of the electrolyte membrane may be slightly thicker than the porous clusters, such that the thickness of the porous clusters is at most 99%, for example 95%, of the thickness of the electrolyte membrane.

Having porous clusters distributed over at least 80% of the thickness of the electrolyte membrane enhances the stability (both mechanical and chemical) of the final electrolyte membrane.

The electrolyte membrane of the invention may comprise more than one porous cluster, for example two porous clusters, distributed over at least 80% of the thickness of the electrolyte membrane.

Fig. 1 shows a schematic view of an electrolyte membrane of the present invention.

The invention also provides a catalysed electrolyte membrane comprising a catalyst layer and an electrolyte membrane of the invention.

The catalyst layer comprises one or more electrocatalysts. The one or more electrocatalysts are independently a finely divided unsupported metal powder, or a supported catalyst in which small nanoparticles are dispersed on a conductive particulate carbon support. The electrocatalyst metal is suitably selected from

(i) Platinum group metals (platinum, palladium, rhodium, ruthenium, iridium, and osmium),

(ii) the metal is gold or silver, and the metal is silver,

(iii) a base metal, a metal oxide,

or an alloy or mixture comprising one or more of these metals or their oxides. The preferred electrocatalyst metal is platinum, which may be alloyed with other noble metals or base metals. If the electrocatalyst is a supported catalyst, the loading of the metal particles on the carbon support material is suitably from 10 to 90 wt%, preferably from 15 to 75 wt%, of the weight of the formed electrocatalyst.

The particular electrocatalyst used will depend on the reaction it is intended to catalyze, and its selection is within the ability of those skilled in the art.

The catalyst layer is suitably applied to the first and/or second face of the electrolyte membrane as an ink, which is organic or aqueous (but preferably aqueous). The ink may suitably comprise other components, such as an ion-conducting polymer as described in EP0731520, which are included to improve the ion-conductivity within the layer. Alternatively, the catalyst layer may be applied by decal transfer of a previously prepared catalyst layer.

The catalyst layer may further comprise additional components. Such additional components include, but are not limited to, catalysts that promote oxygen evolution and thus would be beneficial for cell reversal situations and high potential excursions, or hydrogen peroxide decomposition catalysts. Examples of such catalysts and any other additives suitable for inclusion in the catalyst layer will be known to those skilled in the art.

The present invention further provides a membrane electrode assembly comprising an electrolyte membrane of the present invention and a gas diffusion electrode on the first and/or second face of the electrolyte membrane.

The invention further provides a membrane electrode assembly comprising a catalysed electrolyte membrane according to the invention and a gas diffusion layer present on at least one catalyst layer.

The membrane electrode assembly may be constructed in a number of ways, including but not limited to:

(i) the electrolyte membrane of the present invention may be sandwiched between two gas diffusion electrodes (one anode and one cathode);

(ii) the catalyzed electrolyte membrane of the present invention having a catalyst layer on one side may be sandwiched between a gas diffusion layer and a gas diffusion electrode, the gas diffusion layer being in contact with the side of the catalyzed electrolyte membrane having the catalyst component; or

(iii) A catalysed electrolyte membrane of the invention having catalyst components on both sides may be sandwiched between two gas diffusion layers.

The anode and cathode gas diffusion layers are suitably based on conventional gas diffusion substrates. Typical substrates include nonwoven papers or webs comprising a network of carbon fibers and a thermosetting resin binder (e.g., TGP-H series carbon fiber paper available from Toray industries Inc., Japan, or H2315 series available from Freudenberg FCCT KG, Germany, or SGL Technologies GmbH, Germany)

Series, or available from Ballard Power systems Inc

Series, or woven carbon cloth. The carbon paper, web or cloth may be subjected to additional treatment prior to incorporation into the MEA to make it more wettable (hydrophilic) or more moisture resistant (hydrophobic). The nature of any process will depend on the type of fuel cell and the operating conditions to be used. The substrate may be rendered more wettable by incorporating a material such as amorphous carbon black from a liquid suspension via impregnation, or may be rendered more hydrophobic by impregnating the pore structure of the substrate with a colloidal suspension of a polymer such as PTFE or polyfluoroethylenepropylene (FEP), followed by drying and heating above the melting point of the polymer. For applications such as PEMFCs, a microporous layer may also be applied to the face of the gas diffusion substrate that will be in contact with the electrocatalyst layer. The microporous layer typically comprises a mixture of carbon black and a polymer such as Polytetrafluoroethylene (PTFE).

The invention further provides a fuel cell comprising an electrolyte membrane, a catalysed electrolyte membrane or a membrane electrode assembly as described above. In one embodiment, the fuel cell is a PEMFC.

The electrolyte membrane of the present invention will be used in any electrochemical device requiring such an ion-conducting polymer membrane, such as an electrolytic cell, in addition to being used in a PEMFC.

The invention will be further described with reference to the following examples, which are illustrative and not limiting of the invention.

Example 1

Membrane fabrication

Poly [2,2'- (m-phenylene) -5,5' -bibenzimidazole ] (PBI) obtained from PBI Performance Products inc. was electrospun from a 13% solution of dimethylacetamide (DMAc) using the following parameters: applying voltage at 15kV, and enabling the flow rate to be 0.12 mL/h; the distance between the needle collector and the needle collector is 10 cm; drum collector speed 800 rpm; and a translation speed of 10 mm/s. The electrospun clusters are removed from the drum.

The PBI electrospun cluster comprises randomly oriented nanofibers with an average fiber diameter of 200nm, with a relatively narrow fiber diameter distribution of 140-280nm, and a length of several tens of microns. Fig. 2 provides a Scanning Electron Microscope (SEM) image of the electrospun cluster showing that the fibers are non-randomly oriented. Also shown in fig. 2 is a graph showing the fiber diameter distribution.

The thickness of the PBI nanofibers can be further controlled using coaxial electrospinning. The core solution was PBI solution and the sheath solution was DMAc. By using a solvent sheath, evaporation and drying of the fibers that occurs during electrospinning is delayed, which results in greater stretching of the polymer nanofibers and, ultimately, finer nanofibers in the electrospun cluster. Using a core/sheath flow rate ratio of 2/1, the average fiber diameter was 120 μm (in the range 60-180nm) (see FIG. 3).

The electrospun PBI cluster had a thickness of 10 μm, a porosity of 83%, and a basis weight of 2.27g/m²。

Equivalent weight of 700g/mol from Solvay Specialty Polymers

PFSA dispersion (13% w/v 60/35/5H₂A solution of O/1-propanol/DMAc) was cast onto teflon plates using a doctor blade method. The PBI electrospun cluster was then placed directly on top of the cast PFSA dispersion. Impregnation of the cast PFSA dispersion into the nanofiber cluster was visually confirmed and a second layer of PFSA dispersion was then cast on top of the PBI electrospun cluster. The overall film thickness is controlled by the gate thickness of the doctor blade. The cast electrolyte membrane was first dried at room temperature, then dried at 80 ℃ overnight, and then hot pressed (25 kg/cm) at elevated temperature (160 ℃)²)。

After removal of the solvent and hot pressing, the nominal thickness of the electrolyte membrane was 30 μm, and the electrospun PBI clusters extended at about 85% of the thickness of the electrolyte membrane. The weight ratio of PFSA to nanofibers in the electrolyte membrane was 90: 10.

Fig. 4 gives an SEM image of the electrolyte membrane. Cross-sectional SEM was performed by freeze-cracking the samples in liquid nitrogen. Fig. 4 shows that the electrospun clusters allow some displacement of the fibers during impregnation, resulting in the presence of fibers throughout the cross-section of the electrolyte membrane. This has the benefit of reducing the resistance to proton conduction, since there are no areas within the electrolyte membrane where PFSA is deficient. It also allows greater flexibility of the electrolyte membrane to accommodate swelling-induced mechanical stresses. The invisible separation between the nanofibers and the PFSA matrix after freeze-fracture showed excellent interface between the electrospun clusters and the PFSA. In this electrolyte membrane, the nanofibers appeared to be fully immersed in the PFSA and the interface between the two was indistinguishable, indicating a strong attachment of the PFSA to the nanofiber surface.

Example 2 (comparative example)

A 20 wt% DMAc/acetone solution of Polyethersulfone (PES) was electrospun at 25 ℃ on a rotating and translating drum collector. The nanofiber mats were collected and pressed at 140 ℃. An equivalent weight of 700g/mol from Solvay Specialty Polymers

PFSA dispersion (13% w/v 70/30H₂O/1-propanol solution) was cast onto teflon plates using a doctor blade method. The PES electrospun cluster was then placed directly on top of the cast PFSA dispersion. A second layer of PFSA dispersion was then cast on top of the PES electrospun cluster. The overall film thickness is controlled by the gate thickness of the doctor blade. The cast electrolyte membrane was dried at 50 ℃, then at 120 ℃ and then at 145 ℃.

Example 3 (comparative example)

30 μm unreinforced membrane consisting of

PFSA was prepared with an equivalent weight of 790g/mol (EW 790).

Example 4 (comparative example)

30 μm unreinforced membrane consisting of

PFSA was prepared with an equivalent weight of 700g/mol (EW700), as used in example 1.

The swelling, water uptake and proton conductivity measurements of the membranes were measured at 80 ℃.

In-plane proton conductivity was performed on samples of approximately 25 x 5mm in size using a Bekktech 4-point probe set and a measurement chamber with controlled temperature and Relative Humidity (RH). Impedance measurements were performed at 80 ℃ and 110 ℃ in the 50-95% RH range. Measurements at 110 ℃ were made at a chamber pressure of 206 kPa. The results are given in table 1 as the average of three measurements. The water uptake of the film was determined by weighing a sample (cut using a template) of 3X 3cm size before and after soaking in water at 80 ℃ overnight. The membrane size was swollen on the same sample, determined by measuring the sample size before and after immersion in water.

TABLE 1

The percent swelling in water of the membrane of example 1 is much less than the membranes of examples 3 and 4, which is believed to be due to ionic interactions between electrospun PBI clusters and PFSA ionomers. The proton conductivity of example 1 is higher than that of example 3 and comparable to example 4.

Mechanical tensile strength is measured using the modulus of elasticity and elongation at break. Mechanical tensile measurements were performed on a Zwick roelz1.0 instrument using a 200N static load cell equipped with a controlled humidity/temperature chamber and TestXpert V11.0 software. The test was carried out on a sample of 100X 5mm strips, using a traction rate of 1mm/s and a grip distance of 10 mm. For the measurement at elevated temperature/RH, the sample was kept overnight under the required conditions, then mounted and equilibrated in the sample chamber for 1h, and pre-stretched prior to the measurement to account for any swelling of the membrane. The results are given in table 2.

TABLE 2

The elastic modulus and yield point of the film of example 1 were all significantly higher than the films of examples 3 and 4, which demonstrates that the film of example 1 is stiffer and stronger with a lower elongation at break.

MEA fabrication

MEA 1. membranes and electrodes of example 1 were cut to 52 × 52mm using a template, and sub-cluster circles were used to define 25cm²The region of action of (1). The electrode was a standard electrode with 0.2mg/cm at the anode²And 0.4mg/cm at the cathode²Platinum catalyst loading. The MEA was fabricated by hot pressing at 150 ℃ for 5 minutes.

MEA 2 (comparative): MEA 2 was prepared using the membrane of example 2, following the fabrication method described for MEA 1.

MEA 3 (comparative): MEA 3 was prepared using the membrane of example 3, following the fabrication method described for MEA 1.

MEA 4: MEA 4 was prepared in a similar manner to that described above for MEA 1, using the membrane of example 1, except that the active area was 45cm²And the MEA was fabricated by hot pressing at 170 ℃ for 2 minutes.

MEA 5 (comparative): MEA 5 was prepared using the membrane of example 3, following the fabrication method described for MEA 4.

MEA 6 (comparative): MEA 6 was prepared using the membrane of example 4, following the fabrication method described for MEA 4.

Durability test

Open Circuit Voltage (OCV) fixed component acceleration stress test:

the OCV fixed fraction test was performed under the following conditions to evaluate the durability of the film: the membrane electrode assembly was held at an open circuit voltage at 85 ℃ and 13% RH and the cell voltage was monitored for a decrease over time. As can be seen in fig. 5, MEA 3 showed a significant reduction in OCV, while MEA 1 showed a significantly lesser reduction in OCV.

Wet-dry cycle accelerated stress test:

the wet-dry cycle test was performed at 80 ℃ at OCV by cycling from 0% RH (10 minutes) to 90 ℃ dew point (10 minutes) to further evaluate the durability of the film to volume changes caused by hydration/dehydration. Figure 6 shows that MEA 1 is significantly more stable than MEA 2 and MEA 3. After 150 hours, MEA 2 and MEA 3 showed an increase in OCV retardation rate while MEA 1 remained stable and showed a significantly reduced retardation rate compared to MEA 2 and MEA 3, which shows that nanofiber reinforcement provides mechanical strength and integrity against stress caused by volumetric changes of hydration and dehydration, resulting in significantly improved stability.

Testing the durability of the battery pack:

construct a structure containing 9 working areas of 45cm²The stack is operated under an accelerated durability test protocol designed to replicate the actual duty cycle conditions for fuel cells operating in a real-time environment, the test protocol includes repeated cycles of constant high current operation, followed by cycling between high and low current conditions, then switching from low current conditions to off, and starting to low current conditions (i.e., repeated on-off cycles), rapid cycling between these actual operating modes is designed to accelerate performance characteristics over a shorter time that will be observed during actual fuel cell operation over ten thousandths of an hour of real-time operation, the accelerated test cycle is performed at 50kPag inlet pressure and 30% inlet Relative Humidity (RH), at 80 ℃ stack temperature, for the anode, gases are supplied at 1.5 × stoichiometry, for the cathode, at 2.0 × stoichiometry supply durability test performed almost 2,000 hours, the durability test is performed by running at 0% air, the polarization temperature is measured at 70 ℃ for the stack, at 100.0.0.5% polarization temperature, and the cathode polarization temperature is measured from the ambient air^-2Current density, individual cell voltage versus time. The cell voltages from each of the MEA types MEA 4, MEA 5 and MEA 6 are averaged by 3 MEAs of each type introduced into the short stack.

The average MEA voltage durability for three MEAs is shown in figure 7. It can be seen that MEA 5 and MEA 6 each experienced a significant drop in performance (cell voltage) at about 750 hours, while MEA 4 maintained a very stable cell voltage throughout the 2000 hour test. This fuel cell stack durability test clearly shows the significantly enhanced durability resulting from the MEA using the reinforced membrane of the present invention as compared to other MEAs using non-reinforced, otherwise similar membranes.

In summary, it can be seen from the test results that the MEA of the present invention exhibits improved durability over that exhibited by corresponding unreinforced membranes or by using different types of reinforced membranes.

Without wishing to be bound by theory, the inventors believe that this may be due to phase separation and continuity of the electrospun clusters, and ionic cross-linking (acid-base interactions, or hydrogen bonding) between the ionically conductive polymer and the nanofiber surface in the electrospun web. In addition, the electrospun clusters allow for greater swelling in the thickness direction because the fibers can move relative to each other in this direction, but limit swelling in the in-plane direction because the fibers are not elastic.

In addition, the heterocyclic-based polymer used to form the electrospun cluster has antioxidant properties and can contribute to the stability of the electrolyte membrane by eliminating damaging substances such as peroxide radicals. Due to these antioxidant properties, it may no longer be necessary to incorporate an antioxidant or a hydrogen peroxide decomposition catalyst such as ceria into the membrane.

It is also contemplated that strong non-covalent interactions between the nanofibers and the ionically conductive polymer may allow for reforming or self-healing of the electrolyte membrane after minor damage such as pinholes formed during operation. Such a reforming process will be facilitated by the application of pressure/temperature (conditions already present in the fuel cell during operation).

Claims

1. An electrolyte membrane comprising:

(i) a porous cluster formed from entangled nanofibers, wherein the nanofibers are comprised of a non-ion conducting heterocyclic-based polymer comprising basic functional groups and being soluble in an organic solvent; and

wherein at least 80% of the pores of the porous clusters are filled with an ion conducting polymer, wherein the thickness of the porous clusters in the electrolyte membrane is distributed over at least 80% of the thickness of the electrolyte membrane, and wherein the nanofibers are spun nanofibers.

2. The electrolyte membrane according to claim 1, wherein the heterocyclic-based polymer is selected from the group consisting of polybenzimidazole, polypyridine, polypyrimidine, polybenzothiazole, polyoxadiazole, polyquinoline, polyquinoxaline, polythiadiazole, polytriazole, polyoxazole and polythiazole and derivatives thereof.

3. The electrolyte membrane according to claim 1, wherein the heterocyclic-based polymer is polyazole or a derivative thereof.

4. The electrolyte membrane according to claim 3, wherein the polyazole is selected from the group consisting of polybenzimidazole, polytriazole, polythiazole and polydithiazole and derivatives thereof.

5. The electrolyte membrane according to any one of claims 1 to 4, wherein the average diameter of the nanofibers is 100-400 nm.

6. The electrolyte membrane according to any one of claims 1 to 4, wherein the porosity of the porous cluster is 70 to 98%.

7. The electrolyte membrane according to any one of claims 1 to 4, wherein the average basis weight of the porous clusters is 1.0g/m²-7g/m²。

8. The electrolyte membrane according to any one of claims 1 to 4, wherein the weight ratio of ion-conducting polymer to nanofibers in the electrolyte membrane is greater than 70: 30.

9. A catalysed membrane comprising an electrolyte membrane according to any one of claims 1 to 8 and a catalyst layer on the first and/or second face of the electrolyte membrane.

10. A membrane electrode assembly comprising an electrolyte membrane according to any one of claims 1 to 8 and gas diffusion electrodes on the first and/or second face of the electrolyte membrane.

11. A membrane electrode assembly comprising a catalysed membrane according to claim 9 and a gas diffusion layer on the catalyst layer.

12. A fuel cell comprising an electrolyte membrane according to any one of claims 1 to 8, a catalysed membrane according to claim 9, or a membrane electrode assembly according to claim 10 or 11.

13. The fuel cell according to claim 12, wherein the fuel cell is a PEMFC.

14. An electrolytic cell comprising an electrolyte membrane according to any one of claims 1 to 8.